Plethysmographic apparatus is used for measuring changes in blood volume or flow in an appendage, e.g. arm, leg, etc. In practice, the apparatus comprises a device which is used to artificially occlude one or more blood vessels in the appendage. After the vessels are occluded to their maximum distension, the device effecting the artificial occlusion is rapidly removed and a strain gauge or other apparatus is used to measure the rate at which the artificially occluded vessels return to their former nonartificially occluded state. The rate at which the occluded vessels return to their nonartificially occluded state is a measure of their physical condition and consequently, a measure of the extent of any existing disease or obstruction to which the vessels have been or are being subjected.
In most plethysmographic apparatus, the device used for occluding one or more blood vessels in an appendage is an inflatable cuff bladder contained in a cuff which encircles the appendage. The bladder is inflated with an inflating medium, usually air, until the desired vessels are occluded. After the desired vessels are occluded, the inflating medium is rapidly removed from the bladder and the necessary measurements are made.
In one method used to evacuate the bladder of the inflating medium, a valve, or the like, in a hose or other apparatus used for inflating the bladder is opened to the atmosphere for discharging the inflating medium to the atmosphere. In cases involving severely diseased blood vessels, it is found that the rate of return of the blood vessels to their nonartificially occluded state is low enough that the time it takes to evacuate the bladder and remove the occlusion by simply venting the bladder to the atmosphere is sufficient. However, this is found not to be true in cases involving healthy blood vessels or less severely diseased blood vessels. In those cases, the bladder must be evacuated more rapidly or the occluding effects of the partially evacuated bladder will adversely affect the measurements made.
To evacuate the bladder more rapidly in cases involving healthy or less severely diseased blood vessels, it has become the practice to connect the bladder suddenly to a vacuum, or other low pressure, chamber. With the bladder connected to a vacuum chamber, it has been found that the bladder can be, at least partially, rapidly evacuated at a rate which exceeds the the rate of return of healthy blood vessels to their nonartificially occluded state. A system of the type in which a bladder is evacuated using a vacuum chamber is disclosed in U.S. Pat. No. 4,205,688, granted June 3, 1980.
While the rapid evacuation of a bladder using a vacuum chamber, or the like, has improved plethysmographic measurements to some extent, it has been found that often the bladder collapses near the point of evacuation hose attachment thereto with the result that some of the inflating medium becomes trapped in the bladder thus slowing the release of the constricting effect of the bladder on the appendage.